then you shouldn't provide pdf previews - because these actions are the advantages of the pdfs. Use Image previews instead... but I've never heard of such a usecase. If you can see a document, you can always copy it in some way - even if it's only a screen shot.

One idea would be to have different versions for the same PDF file, one with the "copy text" function disabled (one of the Security options you can enable for a PDF) and the other one with the full permission, then you could make sure, by using permission, that the right users could only see the right version you want them to see.